1. Field of the Invention
This invention relates to a new liquid, storable coating composition which is suitable for the production of highly UV-stable coatings wherein the binder contains a special polyester component having saturated and unsaturated units and, as the curing agent, an amino resin or a blocked polyisocyanate, and to the use of the coating composition for the production of coatings on heat-resistant substrates.
2. Description of the Prior Art
One-component stoving lacquers based on polyisocyanates containing blocked isocyanate groups and their use for the production of polyurethane are known (cf. for example Kunststoff-Handbuch, Vol. VII, Polyurethane, Carl Hanser Verlag Munchen (1966), pages 11-13, 21 et seq).
When blocked polyisocyanates of this type are used in combination with hydroxyl polyesters, the following main requirements have to be satisfied:
1) relatively low unblocking temperatures for short stoving times, PA0 2) little or no yellowing during stoving and in the event of brief overstoving, PA0 3)adequate stability in storage of the one-component lacquers, PA0 4) no elimination of toxicologically harmful decomposition products. PA0 A) 2.5 to 40% by weight, based on the weight of components A) and B), of a crosslinking component containing at least one amino resin and/or at least one blocked polyisocyanate and PA0 B) 60 to 97.5% by weight, based on the weight of components A) and B), of a polyester component containing at least one hydroxyl- and carboxyl-functional polyester, wherein component B) has an average molecular weight (M.sub.n) of 500 to 10,000, a hydroxyl value of 20 to 300, an acid value of 1 to 80 and is based on the reaction product of
These requirements can be satisfied by special stoving binders based on hydroxyl polyesters and blocked polyisocyanates of the type described in DE-OS 2,550,156.
It is apparent from DE-PS 3,046,409 that the stoving binders according to DE-OS 2,550,156 cannot be used for the production of coatings on surfaces for industrial goods which are exposed to impact stresses, such as machine parts, motor vehicle bodies or transport containers.
Although the stoving binders according to DE-PS 3,046,409 provide impact-resistant coatings, they are unable to meet normal requirements in regard to weather stability. Accordingly, they are unsuitable for coating surfaces exposed to extreme weathering influences, such as motor vehicle bodies, facing panels, etc., which have to meet stringent requirements in regard to mechanical properties and resistance to solvents, such as gasoline.
One-component stoving binders based on hydroxyl polyesters and amino resins also do not fully satisfy the stated requirements. However, after stoving they do result in lacquer films which are distinguished by a favorable hardness-to-elasticity ratio and by good adhesion. Binder compositions of this type are described in DE-AS 2,621,657 and may be applied by the automatic coil coating method. The requirements which one-component stoving lacquers, which are based on hydroxyl polyesters and blocked polyisocyanates or amino resins as crosslinking component, have to satisfy today go beyond the described level. Automatic coil coating requires stoving lacquers having weathering properties which are good enough for the pigmented lacquer films to be able to withstand even short-wave UV light for prolonged periods without yellowing or cracking coupled with high surface gloss of the lacquer films. The mechanical properties of the coatings must not show any disadvantages when compared to known films. A high standard is also expected for the abrasion resistance of the lacquer film surface. The use of less solvent is another requirement. The state-of-the-art lacquer systems mentioned above do not fully satisfy these requirements.
It has now been surprisingly found that the coating compositions described in detail hereinafter based on known curing agents and a special polyester component provide lacquer films which not only combine high hardness and elasticity with good adhesion to metal and good abrasion resistance, but also provide lacquer films which withstand exposure to short-wave UV light for more than 1,250 h without cracking or yellowing in the absence of light stabilizers. After stoving, the coating compositions according to the invention result in high-gloss lacquer films which, in addition to excellent adhesion to metallic substrates, also possess high hardness and extreme deformability. The last of these properties is particularly desirable in the case of coating compositions which are applied by automatic coil coating or which are used as non-chip coatings in the automotive field.
The coating compositions according to the invention may be applied in high concentrations and enable environmental pollution to be reduced through the relatively minimal use of solvent.